The present invention generally relates to a multiple sample transport system and, in particular, relates to such a system useful in an apparatus operating under an ultra-high vacuum.
The formation of semiconductor layers on a substrate by use of one or more high temperature molecular beams in a ultra-high vacuum ambient is generally referred to as molecular beam epitaxy (MBE). The significance of MBE is difficult to overstate in the semiconductor manufacturing art since, by use of multiple beams having different impurity contents, the achievable doping profile is extremely controllable and adaptable to the desires of the manufacturer. As such, the resultant semiconductor devices have operating parameters more closely related to the ideal, or theoretical, values than do devices manufactured by the more conventional techniques. Nevertheless, the use of MBE as a viable manufacturing technique is still in its infancy and is on the verge of emerging from the laboratory model to a large scale manufacturing instrument.
As with most scientific advances, the conversion of an apparatus from a laboratory model to a manufacturing model is extremely difficult, time consuming and presents numerous problems. One particular problem in developing a commercial MBE system is that, while in the laboratory, the length of time necessary to process a workpiece, i.e. generally a semiconductor wafer, was a minor consideration; however, such processing times are a major consideration in commercial endeavors. Thus, an apparatus which individually introduces a single workpiece into the vacuum system of an MBE for processing and then removes that workpiece before another can be introduced is quite unacceptable for commercial purposes. This is particularly so, since each time the ultra-high vacuum of the system is broken, a considerable period of time is necessary to re-establish that vacuum.
One solution to this difficulty has been to process a plurality of workpieces simultaneously and thereby reduce the turnaround time. Such a mechanism is described in U.S. Pat. No. 4,137,865 issued on Feb. 6, 1979 to Alfred Y. Cho and assigned to Bell Telephone Laboratories, Inc. Therein, a molecular beam epitaxial apparatus is described wherein a plurality of substrates are conveyed into the growth chamber of the MBE system and, once in the growth chamber, each substrate is individually processed. Such an apparatus must include means within the growth chamber for reducing cross-contamination between the wafers therein. Nevertheless, it is inevitable that some degree of cross-contamination will occur since the substrates are present in the same chamber during the growth of layers on each.
Other possible solutions to the general problems outlined above include the use of various pushrods/bellow arrangements for manipulating wafers within the ultra-high vacuum chamber. Usually these manipulative devices are capable of handling only a single wafer at a time and are quite complex in that they are often magnetically controlled, i.e. controlled by use of a movable magnet which is external to the ultra-high vacuum chamber. As one can anticipate, such mechanisms are quite inaccurate, as well as frustrating and time consuming in the manipulation of a plurality of wafers.
While the apparatus described above has certainly advanced the MBE closer to a commercial system, problems nevertheless are compounded thereby. One such problem is, as mentioned above, the cross-contamination among workpieces within the growth chamber. Another problem is that since all of the substrates are present in the growth chamber, the number of substrates able to be processed at one time is limited by the size of the growth chamber itself. Another difficulty lies in the fact that protective apparatus must be included in the growth chamber to prevent and reduce the cross-contamination. This significantly increases the cost of such a growth chamber.